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Silica Reinforcement of Epoxidised Natural Rubber of Varying Epoxy Content

Jacob K.Varkey*, Sadhan K.De** and K.T.Thomas**Rubber Research Institute of India, Rubber Board, Kottayam, Kerala, India

**Polymer Engineering Department, University of Akron, USA.

International Rubber Conference and IRRDB Meetings28-31 October 2012

Kovalam, Trivandrum

Natural Rubber

Renewable raw material

Excellent mechanical and dynamic properties

Broad range of applications – Major application-Tyres

Mostly used as filled vulcanizates

Carbon black (Petroleum product) and silica (mineral filler) Important reinforcing fillers used in rubber compounds.

Replacement of carbon black by silica in tyre will contribute to low rolling resistance and good wet grip.

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Carbon black and Silica as filler in natural rubber

Carbon black as filler in NR:

Good rubber-filler interaction Low filler-filler interaction Good reinforcement

Silica as filler in NR :

High filler-filler interaction Low rubber-filler interaction Inferior reinforcement

How to improve silica reinforcement of NR?

By the use of Silane coupling agent

By chemically modifying natural rubber so as to makeit polar so that rubber-silica interaction could beincreased.

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Silica –Rubber coupling through silane

Epoxidised natural rubber (ENR)

Chemically modified natural rubber

ENR more polar than NR due to the presence of epoxy group

Rubber-silica interaction is better in ENR than in NR

Silica filled ENR exhibits low rolling resistance and high wet grip

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Natural Rubber

Epoxidised natural rubber

Si OH + C C

O HH3C

H2C CH2

OH O

C

C

CH2

CH2

H3C

H

CH2

C

C

CH2

H

OHCH3

OSi

Si

Silanol ENR

Si OH + C C

O HH3C

H2C CH2

OH O

C

C

CH2

CH2

H3C

H

CH2

C

C

CH2

H

OHCH3

OSi

SiSi OH + C C

O HH3C

H2C CH2

OH O

C

C

CH2

CH2

H3C

H

CH2

C

C

CH2

H

OHCH3

OSi

Si

Silanol ENR

Proposed mechanism for Epoxy- Silanol interaction

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Objective of the study

To compare the silica reinforcement of NR and ENR of varying epoxy content

Materials Used:

Natural Rubber – ISNR 5

Epoxidised natural Rubber: ENR 10, ENR 17.5, ENR 25, ENR 50

Silica – Precipitated silica (Ultrasil VN3 of Degusa AG)

Silane coupling agent – ‘Si 69’ ((bis-triethoxysilylpropyl tetrasulphide)

Zinc oxide, Stearic acid, sulphur, Accelerators, anti oxidants and process oil – rubber grade

EXPERIMENTAL

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a) Rubber used for each experiment was initially masticatedin a laboratory mixing mill to a Mooney viscosity in therange of 65 ± 2.

b) Further mixing of rubber compound was done in threestages in a laboratory model internal mixer

c) Final homogenization of each mix was carried out in thelaboratory mixing mill.

Compound preparation

Testing

Compound testing

Cure characteristics

Using MDR 2000 Rheometer of M/s Alpha Technologies at 150 °C as per ASTM D-2084.

Vulcanizate testingMoulding of test samples –Using an electrically heated hydraulic press to their respective optimum cure time (t90) at 150 °C .Properties of the vulcanizates –Stress-strain properties – ASTM D 412, Tear strength – ASTM D 624 Hardness – ASTM D-2240, Heat build up – ASTM D 623Compression set – ASTM D 395 – Method B, Abrasion resistance – DIN 53516Resilience – ASTM D 1054 Silica dispersion – Using AFM

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Ingredients *

Compound number

1(N/S/S)

2(E10/S/S)

3(E17.5/S/S)

4 (E25/S/S)

5 (E50/S/S)

6(N/S)

7 (E50/S)

NR (ISNR 5) 100 - - - - 100 -

ENR 10 - 100 - - - - -

ENR 17.5 - - 100 - - - -

ENR 25 - - - 100 - - -

ENR 50 - - - - 100 - 100

Silica (Ultrasil VN3)

50 50 50 50 50 50 50

Silane (si 69)

4 4 4 4 4 - -

* Above compounds also contain Zinc oxide 4, Stearic acid 2, N-cyclohexyl-2-benzothiazolesulphenamide (CBS) 1.5, di phenylguanidine (DPG ) 1.5, Sulphur 2, Naphthenic oil 5, Mernox 6C (N-(1,3-dimethylbutyl)-N’-phenyl-p-phenylenediamine ) 2

Formulation of mixes

Cure characteristics

Compound number1

(N/S/S)2

(E10/S/S)3

(E17.5/S/S)4

(E25/S/S)5

(E50/S/S)6

(N/S)7

(E50/S)

Min. Torque, ML,dN.m

1.5 1.3 2.0 2.6 2.1 2.3 2.3

Max. torque, MH, dN.m

19.5 23.6 18.9 21 20.5 32 22.8

Optimum cure time, (t90), min.

8.4 7.5 7.7 6.2 7.6 11.9 8.6

Scorch time,(ts2),min.

4.1 3.5 3.7 2.6 2.5 0.4 3.1

Cure rate index (CRI), 100/t90-t2

23.4 25.3 25.1 27.7 19.5 8.65 18.1

Cure characteristics

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Property Compound number

1(N/S/S)

2(E10/S/S)

3(E17.5/S/S)

4 (E25/S/S)

5 (E50/S/S)

6(N/S)

7 (E50/S)

100%mod., N/mm2

2.2 3.4 3.2 3.4 3.5 1.4 3.2

200%mod., N/mm2

5.7 8.4 8.8 9.7 10.4 2.8 8.8

300%mod., N/mm2

10.4 14.6 15.8 - - 5.3 15.8

Tensile strength, N/mm2

29.2 23 24.5 14.2 14.5 22.2 17.0

Elongation at break,%

595 430 425 260 260 630 320

Stress – strain properties

Stress – strain properties - comparison

Stress - strain properties

0

5

10

15

20

25

30

35

0 100 200 300 400 500 600 700

Elongation, %

Mod

ulus

, N/m

m2

NR/Silica/silane

ENR10/Silica/silane

ENR17.5/silica/silane

ENR25/Silica/silane

ENR50/Silica/silane

NR/Silica

ENR50/Silica

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Variation of tear strength

0

20

40

60

80

100

120

NR/Silic

a/Sila

ne

ENR10/S

ilica

/Sila

ne

ENR17.5

/Sili

ca/S

ilane

ENR25/S

ilica

/Sila

ne

ENR50/S

ilica/S

ilane

NR/Silic

a

ENR50/S

ilica

Tear

str

engt

h, N

/mm

Property Compound number1

(N/S/S)2

(E10/S/S)3

(E17.5/S/S)4

(E25/S/S)5

(E50/S/S)6

(N/S)7

(E50/S)

Hardness, Shore A

64 67 67 71 74 66 71

Resilience,% 67.3 66.6 61.9 57.4 32.8 61.2 54.7

Compression set,%, (70 °C, 22 hrs.)

38.8 41 41 41.2 43.7 50.4 49

Demattia flex -Initiation, (Cycles-lakhs)

2.98 1.96 1.98 0.23 0.10 0.52 0.37

Demattia flex -Failure, (cycles-lakhs)

5.42 3.81 3.39 1.68 0.26 1.76 0.54

Technological properties

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Variation of heat build-up

13

1112

14

16

19 19

0

2

4

6

8

10

12

14

16

18

20

NR/Sili

ca/Sila

ne

ENR10/S

ilica

/Sila

ne

ENR 17.5

/Sili

ca/S

ilane

ENR25/S

ilica

/Sila

ne

ENR50/S

ilica

/Sila

ne

NR/Sili

ca

ENR50/S

ilica

He

at

bu

ild

-up

, d

eg

ree

C

Variation of Abrasion loss, mm3

128 132141

167

192174

196

0

50

100

150

200

250

NR/Sili

ca/Sila

ne

ENR10/S

ilica

/Sila

ne

ENR17.5

/Sili

ca/S

ilane

ENR25/S

ilica

/Sila

ne

ENR50/S

ilica

/Sila

ne

NR/Sili

ca

ENR50/S

ilica

Ab

rasi

on

lo

ss,m

m3

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ATOMIC FORCE MICROSCOPY STUDIES (Phase image)

N/S/S E10/S/S

E17.5/S/S E25/S/S E50/S/S

SAMPLE -1-PHASE -3D SAMPLE – 2 -3D PHASE

SAMPLE -3 – 3D-PHASE SAMPLE 4 -3D – PHASE SAMPLE 5 – 3D PHASE

N/S/S E10/S/S

E17.5/S/S E25/S/S E50/S/S

ATOMIC FORCE MICROSCOPY STUDIES (Phase 3Dimage)

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Conclusions

Cure characteristics of silane modified silica filled NR andENR of varying epoxy content were comparable.

NR/Silica exhibited delayed cure and poor processability,whereas ENR 50/Silica showed reasonale cure rate.

Silane modified Silica filled ENR exhibited a gradualchange in properties with the extent of epoxidation.

Modulus , hardness and compression set of vulcanizatesshowed an increasing trend with extent of epoxidation

Tear strength, elongation at break, abrasion resistance,resilience and flex resistance exhibited a decreasingtrend with extent of epoxidation

NR, ENR10 and ENR 17.5/silica/Silane compositesexhibited low and comparable heat buid-up. Compositeswith higher levels of epoxidation exhibited higher heatbuild-up.

AFM studies revealed that Silica dispersion is better inENR than in NR.

Silane modified silica filled ENR 10 and ENR 17.5exhibited comparatively better overall properties

Conclusions contd……….

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THANK YOU